In plasma arc cutting or welding of metals, a non consumable electrode is recessed within an arc torch nozzle having a constricting outlet orifice. A gas, sometimes referred to as the orifice gas, is passed through the torch bore surrounding the electrode and exits through the constricting orifice. An electric arc is established between the tip of the electrode and the metal workpiece through the constricting orifice. The orifice gas is ionized by the arc to form a plasma and thus issues from the constricting nozzle orifice as a plasma jet. The ionized gas also provides a low electrical resistance path between the electrode and the workpiece. Stated otherwise, a closed electric circuit is maintained by the plasma arc directly between the tip of the electrode and the metal workpiece.
As is well known in the art, it is essential that the tip of the electrode be centered accurately with respect to the constricting outlet orifice. This permits the exiting orifice gas to surround, in a uniform manner, the tip of the electrode and the established electric arc as the gas approaches and issues from the constricting outlet orifice.
If the tip of the electrode is not accurately centered with respect to the constricting outlet orifice, what is referred to in the art as double arcing will occur. This is the jumping of the main current arc from the electrode to the nozzle and then from the nozzle to the workpiece. Double arcing usually destroys the nozzle. Double arcing usually occurs at the start of cutting or welding but may occur during cutting or welding if a properly centered electrode slips from its centered position.
Even if double arcing does not occur, an off center electrode still disrupts the plasma discharged from the constricting outlet orifice and impairs the quality of the cut or weld.
Therefore, those skilled in the art have attempted many approaches to insure that the tip of the electrode is accurately centered with respect to the constricting outlet orifice of a plasma arc torch nozzle. One approach has been to design the various parts of the torch with very close tolerances so that an assembled torch holding an electrode will achieve the required degree of electrode centering. Manufacture of such torch parts to the tolerances required is difficult and expensive. Moreover, even if a torch were manufactured to the required tolerance, the electrode itself may have eccentricities which will cause the tip of the electrode to be off center with respect to the constricting outlet orifice. That is, even though the electrode may be said to be a rigid rod, a given electrode will most likely have eccentricities about a nominal straight center axis.
Because of poor electrode centering, operators have frequently had to resort to rotation of the electrode and the electrode holder and, in some cases, even to bending of the electrode. However, these time consuming procedures have not resulted in sufficiently accurate centering, in many instances, to eliminate double arcing and to produce the desired high quality cuts or welds.
A second approach to the problem is to provide the torch with an adjustable electrode holding device for centering the electrode by the manual adjusting of the holding device. A torch which employs this approach is described and claimed in U.S. Pat. No. 3,069,532 to Hill et al. In U.S. Pat. No. 3,069,532, a collet for gripping and locking an electrode in a selected position is pivoted in the seat of what is referred to as an electrode holder. The point of pivoting is described in U.S. Pat. No. 3,069,532 as being in the lower portion of the torch bore (i.e., near the torch nozzle). The seat has the shape of a cone and the surface of the collet which pivots in the seat has the shape of a cone. That is, the geometric description of the pivot point would be a cone within a cone. The collet is released or locked in its position in the electrode holder seat by frictional force provided by a knob screwed into the upper end of the electrode holder (i.e., at the end of the torch opposite the torch nozzle). When the collet is released by retracting the knob, the collet and hence the electrode may be pivoted about the collet seat located near the bottom of the torch body bore. Thus a skilled operator can adjust the position of the electrode with respect to the center of constricting outlet orifice. After the tip is centered, the operator would then tighten the knob to lock the collet and the gripped electrode in place. Unless long collets were used, it was found that the tightening operation had a tendency to cause the collet to slip from the adjusted position which, in turn, would cause the tip of the electrode to move away from its centered position. The shorter the collet, the greater was the tendency to slip. This is believed attributable to the inability of the collet assembly to adequately lock the electrode in all adjusted positions. Hence, torch design was constrained by the desirability to use long collets.